Time travel and time freezing in Elixir

Question

When writing integration tests that depend on the current date/time, it is very handy to be able to freeze or travel to specific moment (like e.g. timecop for ruby)

Is there a way to achieve something similar in Elixir/Erlang?

I tried mocking Erlang built-ins :os.timestamp, :erlang.universaltime using meck library, however it fails with :cannot_mock_builtin.

In principle I could implement my own utility library than would enable easy mocking of current time and then use it everywhere instead of built-in methods; however, some libraries use built-ins, so this is not a viable option (e.g Ecto.Model.Timestamps, generating inserted_at and updated_at values)

Have you checked the recent changelog for Erlang 18? It has a TImeWarp feature and ferd has written about it in learnyousomeerlang. Erlang time warp documentation and ferd writings on the subject. No sure though they answer your question... — Olinasc
@Olinasc The time warp feature is only related to reacting to changing system time and clocks in general, it does not allow you to change the time for tests only. — Adam Lindberg

Adam Lindberg Adam Lindberg · Accepted Answer · 2015-10-03T10:56:55

Elixir

I would suggest you implement this via dependency injection. For example (using the new time API in Erlang 18):

defmodule Thing do
  def do_stuff(data), do: do_stuff(data, &:erlang.system_time/0)
  def do_stuff(data, time), do: {data, time.()}
end

In your tests, you can easily replace the time code:

defmodule ThingTest do
  use ExUnit.Case

  test "the time" do
    assert do_stuff("data", fn -> 123 end) == {"data", 123}
  end
end

Erlang

Here's the corresponding way to do this in Erlang:

-module(thing).
-export([do_stuff/1, do_stuff/2]).

do_stuff(Data) -> do_stuff(Data, fun erlang:system_time/0).

do_stuff(Data, Time) -> {Data, Time()}.

And the test:

-module(thing_tests).
-include_lib("eunit/include/eunit.hrl").

do_stuff_test() ->
    ?assertEqual({"data", 123}, do_stuff("data", fun() -> 123 end).

Time travel and time freezing in Elixir

1 Answers

Elixir

Erlang